Numerous applications are found commercially today where it is desirable to have a plastic, glass, or other like non-conductive substrate provided with a metal coating on its surface either as a continuous coat or as a patterned or discontinuous coating. Among the applications for such metal coated articles of normally non-conductive materials are circuit boards, automobile hardware, various building and construction hardware, toys, buttons, and the like.
In all such applications the process requires the activation of the non-conductive substrate since electroplating cannot be carried out on such a substrate and electroless plating will also not deposit on such nonconductive surfaces. The activation is followed by an electroless plating which will carry a current for subsequent electroplating or which can alternatively be further electrolessly plated with the same or a different metal.
Commercial prior art activating systems have generally relied upon one or more of the nobel metals, such as palladium. For example, one of the earliest methods of activating such substrates involved a two-step operation involving a first immersion of the substrate in a stannous chloride solution followed by a second immersion in an acid palladium chloride solution. Subsequently a one-step process has been employed commercially, involving a colloidal dispersion of palladium and tin chloride salts as disclosed in U.S. Pat. No. 3,011,920 to Shipley. Still another one-step process is disclosed in U.S. Pat. No. 3,672,923 to Zeblisky which also utilizes noble metals, particularly palladium.
Before activation of the non-conductive substrate, it is generally subjected to various cleaning and etching steps known in the art.
One typical example involving the metal plating of plastics such as acrylonitrile-butadiene-styrene copolymers (ABS) involves the steps of first cleaning the plastic article in a strong alkali bath followed by etching in a chemical etching bath, frequently a chromic acid etch, which serves to enhance adherence of the metal coatings to the surface. Following the etching step, the article is rinsed in water and dipped in hydrochloric acid to neutralize the chromium, rinsed again, and then placed in the activating solution (frequently referred to as a catalyst, seeder or sensitizer). Commonly this is the colloidal dispersion of palladium and tin chloride in accordance with the above-mentioned U.S. Pat. No. 3,011,920. After sensitizing, the article is again rinsed and then placed briefly in an accelerator to remove the tin, rinsed again, and placed in the conventional plating electroless metal plating bath. The noble metal of the activating solution, such as palladium, serves to activate, catalyze, or seed the non-conductive substrate for the subsequent electroless plating bath. After a few minutes in the electroless metal plating bath, the article will have a very thin coating of the selected metal of the bath thereon. It is then rinsed and the article may then be further plated with the same or another metal, either by well-known electroplating processes or by further electroless plating.
The U.S. Pat. No. 3,011,920 to Shipley, referred to above, discloses the use of colloidal dispersions of various metals in combination with reducing agents to achieve activation of non-conductive substrates for subsequent electroless plating. The working examples utilize noble metals or hydrous oxides thereof as the colloidal particles and stannous chloride or stannic acid as a reducing agent. The specification in column 2 refers to the fact that other metals, including numerous nonnoble metals such as copper, may similarly be employed to catalyze non-conductive substrates for electroless deposition.
The U.S. Pat. No. 3,657,002 to Kenney discloses a process for preparing hydrous oxide colloids of many different metals including both noble and non-noble metals for treating or coating non-conducting substrates for subsequent electroless plating.
U.S. Pat. No. 3,993,799 issued to Feldstein also discloses the use of a non-noble metal hydrous oxide colloid for treating non-conductive substrates followed by reduction of the hydrous oxide coating on the substrate to achieve at least a degree of activation for subsequent electroless plating.
U.S. Pat. No. 4,239,538 to Feldstein discloses solutions containing copper ions, stannous ions and a phenol or creosol as a so-called linking agent for treatment of non-conductive substrates for subsequent electroless plating, while Feldstein's U.S. Pat. No. 4,259,376 discloses an emulsion containing copper as the principal catalytic agent and a catalytic promoter consisting of a number of non-noble metals to yield an enhanced catalytic activity for electroless plating of non-conductive substrates.
U.S. Pat. No. 3,958,048 to Donovan discloses a process for the surface activation of non-conductive substrates for electroless plating by treating the surface of the substrate with an aqueous composition containing catalytically active water insoluble particles formed by a reaction of a non-noble metal and a water soluble hydride in the presence of a water soluble organic suspending agent. Copper salts are disclosed as one of the non-noble metals, dimethylamine borane (DMAB) as one of the hydrides, and gelatin as one of the possible organic suspending agents.
The use of copper colloids for the activation of non-conductive substrates in place of the palladium colloids has recently become commercial to a limited extent. With the use of copper activating colloids it has generally been necessary to utilize a fast electroless copper bath in order to obtain good coverage of the non-conductive substrate by the electroless plating step. When the copper activating colloids are utilized and a slow copper electroless bath employed, the degree of coverage of copper by the electroless bath is decreased significantly. For example, when some copper colloids are utilized with fast bath, 100% coverage can be obtained, but when a slow electroless bath is used the coverage obtained may only be on the order of 50 to 75 percent of the surface of the non-conductive substrate. Thus, one could say that the catalytic activity of the copper colloid is sufficient when employing a fast copper electroless bath, but insufficient when a slow electroless copper bath is employed. The applicant has noticed no significant difference in percent coverage between slow and fast electroless copper baths when utilizing the known palladium activating colloids presently in use. These electroless copper baths, fast and slow, are well known within the industry, and can be best characterized both by the speed of the baths and their stability. Generally, however, a fast electroless copper bath would be capable of depositing about 100 micro inches of copper to the desired surface in about 30 minutes, while a slow electroless copper bath would be capable of depositing 35 to 40 micro inches of copper within about 30 minutes. Fast electroless copper baths are unstable and can only be utilized for short periods of time. This instability is generally due to a rapid imbalance of the chemical makeup of the baths during operation. Thus, these fast baths must be frequently checked and reconstituted to the desired chemical balance in order to obtain electrolessly plated substrates suitable for subsequent electroplating and commercial use. This is a cause of great concern in commercial production of such items as circuit boards. A slow copper electroless bath, however, is quite stable and can be used for long periods of time without chemical adjustments, and the use of such bath is highly desired by industry, particularly where only a flash electroless copper coating is desired or a deposit in the range of 35 to 40 micro inches in thickness. This thickness is all that is generally necessary or desired in the production of circuit boards.
There are many variables which determine whether an electroless copper bath is fast or slow. One of the more important factors in determining the speed of such baths is the temperature used during the electroless plating operation and, to a lesser extent, the amount of chemicals utilized to make up such baths. For example, a fast bath can constitute an aqueous solution containing 8 ml/l of a 37% formaldehyde solution, 10 g/l of sodium hydroxide, and 3 g/l of copper metal supplied by a suitable salt, such as copper sulfate. When operating this bath at about 120.degree. F., it is considered to be a fast bath and will deposit about 100 microinches of copper onto a conductive surface in about 30 minutes. An example of a slow bath would be an aqueous solution containing about 20 ml/l of a 37% formaldehyde solution, 15 g/l of sodium hydroxide, and 3 g/l of copper metal; again supplied to the solution by means of a suitable salt. When this bath is operated at about 75.degree. F. or room temperature, it is considered to be a slow bath and it will deposit between about 35 and 40 microinches of copper to a conductive or activated surface in about 30 minutes. When such a slow bath is operated at higher temperatures, such as about 95.degree. F., it becomes a fast electroless copper bath. All of this is known in the art and the terms "slow" and "fast" electroless copper baths are terms of the art.
All of these electroless copper baths also contain stabilizers and complexing agents for the copper. These stabilizing and complexing agents are also well known in the art. The applicant prefers to use divalent sulfur compounds as stabilizing agents, such as those disclosed in the Schneble U.S. Pat. No. 3,361,580, plus a small amount of cyanide ion. The amount of stabilizing agent can be varied in these baths depending upon whether the bath to be employed is a slow or fast bath. Generally it is advisable to increase the amount of stabilizing agent when a fast bath is being employed. This is also well known to those skilled in the art and regulation of the stabilizing agent to obtain optimum stability will depend upon the makeup of the particular bath being employed and the operating temperature of the bath.
The complexing agents are also well known in the art and include such materials as the carboxylic acid type complexing agents, amine carboxylic acid complexing agents, such as EDTA, aliphatic carboxylic acids, such as citric acid, tartrates, and Rochelle salt.